1. Field of the Invention
This invention relates generally to ultrasonic generators, transducers, and converters, and relates more particularly to an ultrasonic transducer or converter having a two-piece head mass or front driver, where one piece provides good thread integrity and the other piece provides good acoustic and/or heat transfer properties.
2. Description of the Relevant Art
Typical prior art stacked ultrasonic transducers or converters 10 and 12 are shown in FIGS. 1 and 2. Both transducers 10 and 12 have multiple PZTs 14 (piezoelectric crystals or transducers), which are annular in shape and are located between a tail mass or back driver 16 and a head mass or front driver 18 (FIG. 1) or 20 (FIG. 2). A bolt 22 is threaded into internal threads in the head mass 18 or 20 to hold the converter together and to compress the PZTs 14 between the head mass and tail mass. An insulating sleeve 23 electrically insulates the PZTs 14 from the bolt 22, and electrical contacts 25 provide electrical connections to the PZTs. A threaded extension 24 connects the converter to a booster or horn (not shown) used for ultrasonic welding or similar application. The PZTs operate in thickness mode, which means they expand and contract primarily in the direction of the central axis 26 of the transducer. The head mass 18 or 20 is tapered in order to amplify the amplitude of the vibrations of the PZTs 14.
In FIG. 1, the head mass 18 is a single material such as aluminum or titanium. Aluminum has an advantage in that it has a high thermal capacity which is useful as a heat sink for transferring heat away from the PZTs. However, aluminum is a relatively soft metal and the screw threads needed to retain the bolt 22 and threaded extension 24 are correspondingly weak. Titanium has superior material strength and thread strength as compared to aluminum, but has a lower thermal capacity and cannot absorb heat as effectively as aluminum.
The transducer 12 shown in FIG. 2 substitutes titanium for aluminum in the threaded area of the head mass. The two-piece head mass 20 is composed of aluminum in the proximal piece 28 next to the PZTs 14 and is composed of titanium in the distal piece 30 that contains internal threads to mate with the bolt 22 and the threaded extension 24. A disadvantage of such a two-piece head mass design is that it does not perform as well as a single-piece head mass (FIG. 1) because having two materials interferes with the amplitude gain of the tapered head mass and the transmission of ultrasonic vibrational energy from the PZTs to the booster or horn.
In other applications, an ultrasonic transducer may be attached to a surface to which ultrasonic vibrational energy is to be transferred. For example, the surface may be the outside surface of a tank holding a cleaning solution and in which objects to be cleaned ultrasonically are immersed. In such an application, the ultrasonic transducer may be adhesively bonded to the tank surface. However, if the material of the tank and that of the head mass are different, there may be a mismatch in the coefficients of thermal expansion, which can cause failure of the adhesive bond. The tank may be made of quartz and the head mass of the transducer may be made of aluminum, which have significantly different coefficients of thermal expansion.